EP0128400B1 - Process for the production of methanol - Google Patents

Description

The invention relates to a process for the production of methanol by reacting hydrogen, carbon monoxide, carbon dioxide and / or water vapor in the presence of catalysts containing copper and zinc.

There are currently two main variants for the production of methanol from synthesis gases, which include Contain hydrogen, carbon monoxide, carbon dioxide and water. The older procedure, also called high-pressure methanol synthesis, uses catalysts based on zinc and chromium oxide. These catalysts are insensitive to a number of catalyst poisons such as e.g. Sulfur and chlorine and show a high resistance to aging. However, the Cr / Zn catalysts are not very reactive, they require high reaction temperatures, for example 320 to 380 ° C, and because of the equilibrium position, high reaction pressures such as. B. 300 to 340 bar.

The newer process variant, also called low-pressure methanol synthesis, uses catalysts containing copper and zinc. These catalysts are much more active, they allow lower reaction temperatures, e.g. B. 220 to 270 ° C, and reduce by the more favorable equilibrium at low temperatures, the technical effort, since this synthesis can be carried out at lower pressures, for example at 50 to 100 bar.

The higher tendency of the catalysts containing Cu / Zn to be deactivated is disadvantageous. There are many reasons for the aging of the catalysts and the individual deactivation mechanisms have largely not yet been clarified. The aging can e.g. B. on a reduction in the number of catalytically active centers by temperature-related recrystallization or by blocking the active centers such as, for example, by reaction with sulfur and chlorine-containing catalyst poisons. Furthermore, a covering of the catalytically active surfaces by other substances is conceivable such as. B. by decomposition products of metal carbonyls or by compounds resulting from competing reactions. Attempts have also been made to reactivate Cu and Zn-containing bifunctional catalysts for the synthesis of dimethyl ether, which also contain chromium and / or high proportions of acidic and dehydrating components, with oxygen-containing gases. The formation of soot or coke-like deposits, as is observed in the synthesis of dimethyl ether carried out at temperatures above 300 ° C in the presence of dehydrating catalysts and which may be regarded as one of the main causes of the deactivation in this synthesis, is generally accepted for LP methanol synthesis carried out at low temperatures is of no importance (Ullmann, 4th edition, vol. 16, p.627).

Attempts have also already been made to increase the service life of copper and zinc-containing LP methanol catalysts by pre-cleaning the synthesis gas used. In addition, numerous proposals are known for increasing the thermostability of the catalysts by adding promoters.

Nevertheless, a not inconsiderable aging of the LP methanol catalysts remains, which cannot be prevented by any of the known measures and which makes it necessary to replace the catalyst after a certain period of time. This procedure is not only costly because of the replacement of the catalyst, it is also technically complex and time-consuming because of the measures associated with the removal of the aged catalyst and reinstallation of the fresh contact. Regeneration after decrease in reactivity, e.g. by treatment with oxygen, i.e. by burning off the coke deposits required the time-consuming measures of removing and reinstalling the catalyst to be regenerated.

It is therefore the object of the present invention to develop a method which allows the consequences of the at least partially methanol-specific aging to be completely or partially eliminated and the original activity of the Cu / Zn contact to be restored completely or at least to a considerable extent.

In particular, the task was to coordinate the process management from the introduction of the catalyst and reduction to the ready-to-use state, control of the course of the reaction to ensure optimum duration and regeneration.

It has now been found that the process for the production of methanol by catalytic reaction of synthesis gas mixtures containing hydrogen, carbon monoxide, carbon dioxide and / or water at temperatures between 200 and 320 ° C and pressures between 30 and 300 bar in adiabatic and / or isothermal led reactors in the presence of a copper and zinc-containing catalyst can be optimized in the desired manner if the fresh catalyst is first reduced with a hydrogen-containing gas under normal pressure or slightly elevated pressure at temperatures rising from 150 to 250 ° C., Reduction is continued until the formation of reduction water significantly subsides and only then is the synthesis initiated under known conditions and continued until there is a significant decrease in the formation of methanol in the reaction zone, the reaction is interrupted and the catalyst then immediately the interruption of the reaction is flushed with inert gases at temperatures of 10 to 300 ° C and regenerated in situ at normal or moderately elevated pressure by passing oxygen-containing gases at temperatures of 150 to 200 ° C over the catalyst, the oxygen content throughout the course the regeneration of the respective temperature is adjusted and the

Transfer of the oxygen-containing gases is continued until the peak of the temperature profile has passed through the entire catalyst.

As inert gas for the purging, e.g. Suitable for nitrogen or methane. If the catalyst is flushed with methane, it is advisable to switch to nitrogen flushing before the regeneration in order to remove the last residues of methane from the catalyst zone before the oxygen necessary for the regeneration is introduced.

When carrying out the process, the fresh catalyst is expediently placed under normal pressure or slightly elevated pressure at a temperature rising from 150 to 250 ° C. by passing a hydrogen-containing gas, for. B. nitrogen-hydrogen mixtures or nitrogen synthesis gas mixtures, reduced. When starting up, the temperature is expediently increased from 150 to 180 ° C and only when the formation of water of reaction, i.e. after the main reaction, the temperature slowly increases, e.g. up to 230 ° C.

The synthesis can then be initiated under conditions known per se.

The LP methanol synthesis is carried out at pressures of 30 to 300 bar, preferably 40 to 120 bar and at temperatures between 200 and 320 ° C, preferably 230 to 280 ° C, under almost isothermal and / or adiabatic conditions. The catalyst used is copper and zinc-containing catalysts with copper contents between 8 to 70% by weight of CuO, preferably 15 to 60% by weight of ZnO, and the additional metal compounds of the second and third main groups of the periodic table, such as magnesium and aluminum and / or the third to seventh subgroup such as lanthanum, thorium, vanadium, chromium and manganese in amounts of 0 to 50 wt .-% metal oxide. Catalysts with 0 to 40% by weight Al ₂ O ₃ , advantageously 1 to 8% by weight A1 ₂ 0 ₃ and / or 0 to 35% by weight Cr ₂ 0 ₃ , advantageously 1 to 15% by weight, are preferred. % Cr ₂ 0 ₃ and / or 0 to 15 wt .-% V ₂ 0 ₅ , advantageously 2 to 10 wt .-% V ₂ 0 _{5 used} as an additive. The catalysts can be prepared by precipitation from aqueous solutions of corresponding metal salts or else by impregnation of an essentially catalytically inert support and subsequent drying and calcination. In order to avoid dimethyl ether formation, it is advantageous not to add any dehydrating components to the catalysts, e.g. B. zeolites and / or y-A1 ₂ 0 _3rd A suitable catalyst is, for example, the Cu / Zn / Al catalyst described in Example 1 of DE-PS 2846614.

The advantage of the process according to the invention is that, without exchanging the catalyst with filling and refilling in the reactor, extensive reactivation is achieved, which can be repeated one or more times depending on the procedure, and thus considerably extends the overall service life of the catalyst. Repeated regeneration is also carried out in the manner described.

The process according to the invention is explained in more detail in the following examples.

example 1

An LP methanol catalyst containing 36% by weight of CuO, 48% by weight of ZnO and 3% by weight of Al ₂ O ₃ , which is compressed to 5 x 5 mm pills with the addition of 2% by weight of graphite installed in a quasi-isothermally operated tubular reactor and reductively activated at 180 ° C in a single pass without pressure using a hydrogen-nitrogen mixture (1 vol.% H ₂ ) at a gas load of 300 to 400 Ni / kg - h. The course of the activation is followed by the formation of the reduction water. After the water content in the reactor discharge has subsided and the H ₂ / N ₂ supply remains constant, the temperature is gradually raised from 180 ° C. to 230 ° C. within 2 to 3 hours and then the hydrogen content of the reactor inlet gas is raised from 1% within 4 to 5 hours 100% increased.

The reactor is operated at 50 bar and 250 ° C with a metal carbonyl-containing synthesis gas (71% H ₂ , 19% CO; 10% CO ₂ ). After several weeks of operation, the activity of the catalyst, expressed as crude methanol production, drops to 79% of the initial value.

The damaged contact is regenerated after the reactor system has been flushed with nitrogen by charging the reactor with an oxygen-containing regeneration gas (nitrogen with 0.2 to 0.5% O ₂ ) at atmospheric pressure in a single pass. The gas load is 3500 Nl / kg · h. During the regeneration, the reactor temperature is kept constant at 150 ± 5 ° C to avoid excess temperatures. After 24 h, the catalyst is reductively activated in the manner described above. After exposure to the synthesis gas characterized above, it is shown that its activity, measured as a raw methanol balance, rises to 88% of the initial value. The rate of deactivation of the regenerated catalyst is not greater than that of the fresh catalyst. Removal samples show damage caused by the iron and nickel carbonyls contained in the synthesis gas as a catalyst poison, particularly in the gas entry zone compared to fresh catalyst with increased iron and nickel contents.

Example 2

The catalyst used in Example 1 is reductively activated in the same way and then with a synthesis gas of H ₂ , CO and CO ₂ (molar composition 75: 20: 5), which contains slight traces of catalyst poisons in the form of nickel carbonyl, in one quasi-isothermal tubular reactor at 50 bar and 260 ° C. After the activity of the catalyst, determined as a raw methanol balance, has dropped to 88% of the initial value, the catalyst is regeneratively oxidized, as described in the first example, by 68 hours with a 0 ₂ / N ₂ mixture (0.2 to 0.5% 0 ₂ ) is treated at 150 ° C under atmospheric pressure. Despite the removal of approximately 3% of the catalyst mass, an increased amount of crude methanol compared to the production before regeneration is produced after the subsequent reductive activation. It is 94% of the initial value. Removal samples of the catalyst show an increased nickel content at the reactor inlet.

Example 3

A technical low-pressure methanol catalyst with 36% by weight CuO, 48% by weight ZnO and 3% by weight A1 ₂ 0 ₃ is operated in an almost isothermally operated technical reactor, which is the core of a complete methanol synthesis cycle, in the cycle mode by reduction in its transferred active form. Hydrogen is used as the reducing agent. Before activation begins, the circuit is flushed with nitrogen and filled and an overpressure of 4 bar is set. The cycle gas is warmed up to 180 ° C. The reductive activation is started by feeding enough hydrogen into the cycle gas that the H ₂ concentration at the reactor inlet is at values of ≤ 0.5 vol% H ₂ . After the start of the reaction, which can be recognized by a slightly elevated temperature (AT <10 ° C) in the top catalyst layer and by the occurrence of reducing water instead of hydrogen in the reactor exit gas, the hydrogen feed is increased so that the theoretically required H ₂ Quantity is supplied within one day and at the same time the hydrogen content in the reactor inlet gas does not exceed 1 to 1.5% by volume by the choice of the circulating gas quantity. The course of the reduction is controlled via the catalyst bed temperatures, the H ₂ and H ₂ 0 concentration in the reactor outlet gas and the amount of water of reduction. If temperature peaks occur in the migrating reduction zone that are more than 10 ° C above the average bed temperature, the H ₂ feed is throttled or interrupted until the temperature falls below this limit again. The hydrogenation in the reduction zone converts almost completely to water which appears in the reactor outlet gas instead of the hydrogen. During the reductive activation, in addition to reducing water, carbon dioxide is also generated, the accumulation of which in the synthesis cycle is kept to less than 15 vol.% CO ₂ by discharging cycle gas.

• 1. die durch eine leicht erhöhte Temperatur gekennzeichnete Reduktionszone das Katalysatorbett durchwandert hat,
• 2. der Gehalt an Reduktionswasser im Reaktoraustrittsgas abfällt und damit
• 3. der Wasserstoffgehalt im Reaktoraustrittsgas ansteigt.

The first phase of reductive activation is finished when

• 1. the reduction zone characterized by a slightly elevated temperature has passed through the catalyst bed,
• 2. The content of reduction water in the reactor outlet gas drops and thus
• 3. the hydrogen content in the reactor exit gas increases.

The post-reduction, which serves to activate any locally reduced non-reduced catalyst components, is initiated by gradually increasing the catalyst bed temperature by 10 to 20 ° C / h to 220 to 230 ° C. After the hydrogen concentration in the reactor outlet gas has been adjusted to that in the reactor inlet gas, the hydrogen feed is regulated in such a way that the hydrogen concentration in the cycle gas doubles approximately every 2 hours. When 30 vol.% H _{2 is reached} in the cycle gas, the post-reduction phase is complete.

ausdrückt.Fresh gas is taken over at 230 ° C. With the metal carbonyl-free fresh gas fed in, which contains 68% by volume of H ₂ , 16% by volume of CO, 12% by volume of CO ₂ , 0.04% by volume of H ₂ 0 and the remainder inert (CH ₄ , N ₂ ), the Methanol synthesis circuit pressed at 15 bar / h to a reaction pressure of 75 bar. After the onset of methanol formation, the catalyst bed temperature is raised to 245 ° C. The methanol synthesis is carried out under these conditions, a fresh gas to reactor inlet gas ratio of 6 kg / kg and a fresh gas load of 0.5 tIt _cat . h operated until the specific methanol production has dropped to 80% of the initial value. This methanol production corresponds to a catalyst activity of 26% of the initial state if the catalyst activity as the reaction rate constant k _{o of} a kinetic model of the form (r = reaction rate)

expresses.

The reaction is then interrupted by relaxing the synthesis circuit at 15 bar / h to a pressure of 2 bar and lowering the catalyst bed temperature to 220 to 230 ° C. Immediately afterwards, the synthesis cycle is flushed several times with nitrogen. The purging takes place in the cycle operation in such a way that nitrogen is injected into the synthesis cycle up to 10 bar within one hour and then expanded again to 2 bar in the same period. After the first rinse, the catalyst bed temperature is lowered to 170 ° C. The nitrogen purge is continued until hydrogen and CO can no longer be detected in the cycle gas (<1% by volume).

Regeneration is initiated by feeding air into the nitrogen-filled circulatory system at 170 ° C. The pressure at the reactor inlet is 2 to 5 bar and the oxygen content of the reactor inlet gas is not more than 0.5% by volume. The air volume in the start phase of regeneration is 2 to 3 Nm ³ air / tcat. H. After initiation of the regeneration, to be recognized by a slightly elevated temperature (.DELTA.T ≦ 10 ° C) in the uppermost catalyst layer, the air feed is within an hour to 5 Nm ³ of air / t _cat · h, and then after a further hour within a Hour to 10 Nm ³ air / tcat. h increased. After the reaction has started, the oxygen content in the reactor inlet gas is kept at values of less than 1% by volume, that of the reactor outlet gas should not exceed values of 0.2% by volume in the first hours of regeneration. When increasing to higher values, in in both cases the air supply is interrupted. The course of the regeneration is controlled by passing through the reaction zone with an increase in temperature of OT <_ 10 ° C compared to the mean bed temperature. If temperature peaks of Δτ> 10 ° C occur, the air feed is throttled or interrupted until the temperature falls below this limit again. The formation of carbon dioxide is observed during the regeneration. The pressure of the reaction system is kept constant (2 to 5 bar) by discharging the cycle gas.

• 1. die durch eine leicht erhöhte Temperatur gekennzeichnete Reaktionszone das Katalysatorbett durchwandert hat und
• 2. der Sauerstoffgehalt des Reaktoraustrittsgases mehr als 80% des Sauerstoffgehalts des Reaktoreintrittsgases beträgt.

The main phase of regeneration has ended when

• 1. the reaction zone characterized by a slightly elevated temperature has migrated through the catalyst bed and
• 2. the oxygen content of the reactor outlet gas is more than 80% of the oxygen content of the reactor inlet gas.

To regenerate any locally non-regenerated catalyst components, the air is then fed in with 20 Nm ³ air / tcat. h continues until the oxygen concentrations in the reactor inlet gas and reactor outlet gas have equalized and are at least 10% by volume of O ₂ . Then, in preparation for the reductive activation, the catalyst bed temperature is raised to 180 ° C. at 10 ° C./h and, at the same time, the synthesis cycle is flushed with nitrogen, as indicated above, until the oxygen content in the cycle gas has dropped to values less than 0.2% by volume. Then, at an overpressure of 4 bar, the regenerated catalyst is activated reductively in cycle operation as indicated above and the fresh gas characterized above is then fed into the synthesis cycle. The activity of the regenerated catalyst after restarting is 86% of the initial value with fresh catalyst, if the activity is given in the form of the reaction rate constants given above. The specific methanol production is then 93% of the initial value under the conditions specified above.

Claims (1)

1. A process for the preparation of methanol by catalytic conversion of a synthesis gas mixture, containing hydrogen, carbon monoxide, carbon dioxide and/or water, at from 200 to 320°C and under from 30 to 300 bar in adiabatic and/or isothermal reactors in the presence of a catalyst containing copper and zinc, wherein the fresh catalyst is first reduced with a hydrogen-containing gas before the start-up of the process, under atmospheric or slightly superatmospheric pressure at temperatures increasing from 150 to 250°C, the reduction being continued until the formation of water from the reduction reaction substantially declines, and then the synthesis is started under conventional conditions and is continued until the formation of methanol in the reaction zone has declined substantially, after which the reaction is interrupted and immediately thereafter the catalyst is flushed with an inert gas at from 10 to 300°C and is regenerated in situ under atmospheric or moderately superatmospheric pressure by passing an oxygen-containing gas over the catalyst at from 150 to 200 °C, the oxygen content being adapted to the particular temperature during the entire course of the regeneration, and the passage of the oxygen-containing gas being continued until the peak of the temperature profile has moved across the entire catalyst.